1. Field of the Invention
The invention relates generally to the field of wellbore instruments and well logging methods. More specifically, the invention relates to systems and methods for estimating permeability of subsurface rock formations using electrical resistivity measuring instruments
2. Background Art
Well logging instruments are devices configured to move through a wellbore drilled through subsurface rock formations. The devices include one or more sensors and other devices that measure various properties of the subsurface rock formations and/or perform certain mechanical acts on the formations, such as drilling or percussively obtaining samples of the rock formations, and withdrawing samples of fluid naturally present in the pore spaces from the rock formations. Measurements of the properties of the rock formations made by the sensors may be recorded with respect to the instrument axial position (depth) within the wellbore as the instrument is moved along the wellbore. Such recording is referred to as a “well log.”
Well logging instruments can be conveyed along the wellbore by extending and withdrawing an armored electrical cable (“wireline”), wherein the instruments are coupled to the end of the wireline. Such conveyance relies on gravity to move the instruments into the wellbore. Extending and withdrawing the wireline may be performed using a winch or similar spooling device known in the art. It is also known in the art to use “logging while drilling” (“LWD”) instruments in certain circumstances. Such circumstances include expensive drilling operations, where the time needed to suspend drilling operations in order to make the wellbore accessible to wireline instruments would make the cost of such access prohibitive, and wellbores having a substantial lateral displacement from the surface location of the well. Such circumstances can also include large lateral displacement of the wellbore particularly where long wellbore segments having high inclination (deviation from vertical). In such cases, gravity is not able to overcome friction between the instruments and the wellbore wall, thus making wireline conveyance impracticable. LWD instrumentation has proven technically and economically successful under the appropriate conditions. LWD instrumentation has also proven quite valuable for determining the position of the wellbore with respect to certain types of rock formations during the drilling of the wellbore, such that the wellbore may be drilled to penetrate certain selected rock formations while avoiding others. Such placement is facilitated by transmission of certain LWD measurements to the surface during wellbore drilling operations. By interpreting the measurements made during drilling, the wellbore operator may make suitable adjustments to the wellbore trajectory to maintain the wellbore within selected rock formations.
The use of LWD instruments has also made possible the determination of the condition of certain permeable subsurface rock formations prior to substantial displacement of the originally present fluid disposed in the pore spaces of the rock formations by the liquid phase of fluid used to drill the wellbore. As is known in the art, typical wellbore drilling operations include pumping a liquid having solid particles suspended therein through the pipe string used to drill the wellbore. The suspension performs the functions of maintaining a selected hydrostatic pressure in the wellbore to prevent entry of fluids from the surrounding formations, to maintain mechanical integrity of the wellbore, to cool and lubricate the drill bit as it drills through the rock formations, and to lift the drill cuttings to the surface for treatment and disposal. In order to prevent entry into the wellbore of formation fluids, the density of the drilling fluid is usually selected to provide hydrostatic pressure somewhat greater than the fluid pressure in the pore spaces of permeable subsurface rock formations. A result of such conditions is that the liquid phase of the drilling fluid is displaced into the pore spaces of the formations, in a process called “invasion.” At the time wireline wellbore instruments are typically operated, the invasion process has reached equilibrium, that is, a filter cake has deposited on the wellbore wall adjacent to the permeable formations, and little additional liquid phase of the drilling fluid enters the pore spaces of the permeable formations. Wireline electrical resistivity instruments typically include devices that have relatively short axial resolution, and have lateral (radial) response generated laterally proximate the wellbore. Such devices may be combined with other devices that have successively greater lateral response and larger (coarser) axial resolution. Measurements from such combined devices may be processed to provide a result that is representative of the electrical resistivity laterally deep enough into the formation such that there is substantially no effect of the liquid phase of the drilling fluid (the “uninvaded zone”). The results may include an estimate of electrical resistivity of the formation laterally proximate the wellbore such that the electrical resistivity is representative of having some of the mobile original or “native” fluid (i.e., the fluid present in the rock pore spaces prior to any effects caused by drilling) moved by the liquid phase of the drilling fluid (the “flushed zone”).
When using LWD instrumentation, the foregoing types of measurements may be made at a time so close to the initial penetration of the rock formation by the drill bit, that relatively shallow invasion has taken place. Thus, the relative lateral dimensions of the flushed zone and the uninvaded zone may be different than those measured at the time of wireline well logging. It is also known in the art to move LWD instrumentation past previously drilled formations one or more times during certain drilling operations. For example, when reinserting the drill string into the wellbore after a drill bit is changed, or when “reaming” or “washing” the wellbore in order to improve its mechanical condition, the LWD instruments may be moved past previously drilled formations and may make measurements at such times. The drilling process also can have periods of time where the LWD instrumentation is stationary in the wellbore, e.g., such as when an additional section of drill pipe is added to the drill string at the surface. The LWD instrumentation may be configured to continue to make measurements of the formation in front of the sensor during these stationary times. Such repeated measurements and continuing stationary sensor measurements may provide a basis to estimate permeability of the formations penetrated by a wellbore.